Organic light-emitting diodes (OLED) generate light by re-combination of electrons and holes, and emit light when a bias is applied between the anode and cathode such that an electrical current passes between them. The brightness of the light is related to the amount of the current. If there is no current, there will be no light emission, so OLED technology is a type of technology capable of absolute blacks and achieving almost “infinite” contrast ratio between pixels when used in display applications.
Several approaches are taught in the prior art for pixel thin film transistor (TFT) circuits to deliver current to an element of a display device, such as for example an organic light-emitting diode (OLED), through a drive transistor. In one example, an input signal, such as a high “SCAN” signal, is employed to switch transistors in the circuit to permit a data voltage, VDAT, to be stored at a storage capacitor during a programming phase. When the SCAN signal is low and the switch transistors isolate the circuit from the data voltage, the VDAT voltage is retained by the capacitor and this voltage is applied to a gate of a drive transistor. With the drive transistor having a threshold voltage VTH, the amount of current to the OLED is related to the voltage on the gate of the drive transistor by:
      I    OLED    =            β      2        ⁢                  (                              V            DAT                    -                      V            OLED                    -                      V            TH                          )            2      
TFT device characteristics, especially the TFT threshold voltage VTH, may vary with time or among comparable devices, for example due to manufacturing processes or stress and aging of the TFT device over the course of operation. With the same VDAT voltage, therefore, the amount of current delivered by the drive TFT could vary by a large amount due to such threshold voltage variations. Therefore, pixels in a display may not exhibit uniform brightness for a given VDAT value.
Conventionally, therefore, OLED pixel circuits have high tolerance ranges to variations in threshold voltage and/or carrier mobility of the drive transistor by employing circuits that compensate for mismatch in the properties of the drive transistors. For example, an approach is described in U.S. Pat. No. 7,414,599 (Chung et al., issued Aug. 19, 2008), which describes a circuit in which the drive TFT is configured to be a diode-connected device during a programming period, and a data voltage is applied to the source of the drive transistor.
The threshold compensation time is decided by the drive transistor's characteristics, which may require long compensation time for high compensation accuracy. For the data programming time, the RC constant time required for charging the programming capacitor is determinative of the programming time. As is denoted in the art, the one horizontal (1H) time is the time that it takes for the data to be programmed for one row.
With such circuit configuration as in U.S. Pat. No. 7,414,599, the data is programmed at the same time as when the threshold voltage of the drive transistor is compensated. It is desirable, however, to have as short of a one horizontal time as possible to enhance the responsiveness and operation of the display device. This is because each row must be programmed independently, whereas other operations, such as for example drive transistor compensation, may be performed for multiple rows simultaneously. The responsiveness of the display device, therefore, tends to be dictated most by the one horizontal time for programming. When the data is programmed during the same operational phase as the drive transistor is compensated, the one horizontal time cannot be reduced further due to compensation accuracy requirements for the drive transistor, as the compensation requirements limit any time reductions for the programming phase.
Another approach is described in U.S. Pat. No. 7,277,071 (Choon-Vul Oh, issued Oct. 2, 2007). In such circuit, two capacitors are used, one for storing the programmed data voltage and the other one for storing the threshold voltage of the drive transistor. In this way, the one horizontal time is dictated by the data programming time only, but this scheme does not use the advantage of independent programming time to minimize the one horizontal time. The configuration uses the same scan signal in the consecutive rows for threshold voltage compensation and data programming. Hence, the one horizontal time is still dependent upon the threshold compensation time.
Other approaches to address the above problems have proven deficient. U.S. Pat. No. 7,317,435 (Wei-Chieh Hsueh., issued Jan. 8, 2008) describes a similar scheme of using two capacitors, one for storing the programmed data voltage and the other one for storing the threshold voltage. The data programming time is reduced as compared to the threshold compensation time. The data line, however, also supplies a reference voltage for “Reset” and “Compensation” phases, and the data line thus cannot be used by other rows during the above two phases. Accordingly, the effective one horizontal time is not actually reduced in a meaningful manner over conventional configurations.
U.S. Pat. No. 9,455,311 (Hideaki Shishido, issued Sep. 27, 2016) describes a scheme with a longer the threshold compensation time and a shorter data programming phase performed at the end of the threshold compensation phase. Hence, a shorter one horizontal time is achieved. A disadvantage of this scheme, however, is that when data is programmed, the programming operation can disturb the compensated threshold voltage and compromise the accuracy of threshold compensation. U.S. Pat. No. 9,773,449 (Yung-Ming Lin, issued Sep. 26, 2017) describes a similar scheme by which data is programmed after threshold voltage compensation. The data programming does not disturb the threshold compensation, but a disadvantage of this scheme is that the terminal of the capacitor for data programming is floating during the emission phase. The noise from the data line could couple to the programming capacitor, so the gate voltage of the drive transistor could be affected by this noise during the emission phase. Hence, the OLED current could be disturbed by the noise from the data line.